The communication technique known as spread spectrum (SS) was developed during World War II with the primary intent of protecting military and diplomatic communications. Spread spectrum communication techniques differ from conventional narrow-band communication techniques because they spread, rather than concentrate, transmitted signals over a wide frequency range. In other words, spread spectrum communication systems effectively spread a narrow-band information signal into a corresponding wide-band signal that closely resembles background radio frequency (RF) noise. Such noise-like characteristics are one of the great advantages of spread spectrum communication systems. That is, because spread spectrum signals are noise-like, they are difficult to detect and hence, there is an inherently high degree of security with SS type communication techniques. Consequently, SS has been and remains the communication technique of choice for many military applications. Without going into great detail, it should also be appreciated that, for similar reasons, SS signals are also difficult to intercept and even more difficult to jam or interfere with than conventional narrow-band signals. Again, such exceptional low probability of intercept (LPI) and anti-jam (AJ) characteristics are why the military has used SS based communication systems for so many years.
Spectrum signals are so wide, they transmit at a much lower power spectral density (Watts per Hertz), than conventional narrow-band transmitters. This lower transmitted power spectral density characteristic is another significant advantage of SS communication systems, as SS and narrow band signals can occupy the same band, with little or no interference. Consequently, SS communication systems exhibit a high degree of immunity to interference generated by other equipment. As a result of this interference immunity, the Federal Communications Commission and other national and international regulatory agencies allow RF equipment to transmit at higher power levels (i.e., longer range transmission) if spread spectrum transmission techniques are employed. Hence, there is significant commercial interest in SS communication systems today.
The expansion or widening of bandwidth in SS type communication systems is accomplished through the implementation of a pseudo-random sequence of binary information, known as a spreading code. The random quality of the spreading code is ultimately responsible for the noise-like appearance of the transmitted broadband SS signal. In reality, the binary sequence that comprises the spreading code is predictable, and consequently does repeat (hence the “pseudo” term). However, the randomness of the code is sufficient to minimize the possibility of accidental duplication or discovery, and as such the spreading code functions much like a security encryption key.